A nut allergy is an adverse immune response to nuts or food containing nuts. When a person has a nut allergy, their immune system mistakenly recognizes proteins in nuts as foreign substances and produces antibodies against them. This can cause a range of symptoms from mild to severe, including itching, hives, difficulty breathing or swelling of the face, tongue or throat. A nut allergy is considered a serious medical condition that requires careful management by an allergist and avoidance of nuts in one's diet.